1. Field of the Invention
The present invention relates to a process for forming a gold crystal film, and more particularly to a process for forming a gold crystal thin film suitable for the production of electronic devices, optical devices, piezoelectric devices, etc. of semiconductor integrated circuits, optical integrated circuits, magnetic circuits, etc., or electron-emitting devices, electrodes for recording media, etc.
2. Related Background Art
Heretofore, gold thin films have been widely used in decoration of accessories such as watchbands, etc., wires or electrodes in the thick film field such as lead frames, HIC, etc., or thin film wires for GaAs semiconductors. Recently electromigration of Al wires has been a serious problem due to requirements for higher density of Si semiconductor devices. Thus, gold as a heavy metal has been regarded as a promising wire material. Particularly a gold crystal thin film comprising a group of monocrystals of large grain sizes, which has excellent anti-electromigration and anti-corrosion, a low resistance, a good anti-melting, etc. has been in demand.
From this background attempts have been made to develop processes for forming a gold crystal film. A vacuum vapor deposition process, a CVD process, an electrolytic plating process, and an electroless plating process are known as processes for forming a gold crystal thin film on a substrate.
The vacuum vapor deposition process comprises heating a substrate of Si, SiO.sub.2, SiN, GaAs, sapphire, Cr, Ti, or Cu to a temperature of 500.degree. to 700.degree. C. by electron beam or resistance heating, and then introducing gold in a vapor phase thereto under vacuum, usually, of not more than 10.sup.-6 Torr, thereby depositing the gold onto the substrate. It is reported that monocrystals of gold having such large grain sizes as a few 10 .mu.m can be formed on mica by the vacuum vapor deposition process (Dennis J. Trevor et al: Physical Review Letters, Vol. 162, No. 8, Feb. 20, 1989).
It is also reported that a gold polycrystalline thin film can be formed on Si by a hot CVD process and a PECVD process (N. Misawa et al: No. 37 Symposium on Semiconductor-Integrated Circuit Technology, Dec. 7, 1989).
The electrolytic plating process and the electroless plating process are old processes utilized in formation of decorating gold thin films or plating of lead frames and comprise forming a layer of gold, copper or nickel on the surface of a substrate in advance, and then forming a gold thin film thereon.
Furthermore, a technique of depositing gold powder for an electroconductive gold paste in a suspended state in a system by utilizing a gold supersaturation phenomenon in the solution due to decomposition of gold complexes is disclosed in Japanese Patent Applications Kokai (Laid-open) Nos. 56-38406 and 55-54509.
According to the vacuum vapor deposition process and the CVD process, a gold crystal thin film composed of a group of monocrystals having large grain sizes can be formed on an inert substrate of mica or highly oriented graphite, but only a gold thin film composed of a group of monocrystals having submicron grain sizes or a gold polycrystalline film is formed on a semiconductor material of Si or GaAs or on ceramics of SiO.sub.2, SiN or Al.sub.2 O.sub.3.
Furthermore, the vacuum vapor deposition process and the CVD process require an elevated substrate temperature and thus devices incapable of being disposed at an elevated temperature suffer from many restrictions. For example, in case of forming a gold thin film on Si, an Au--Si alloy is formed by melting and thus the substrate temperature can never be more than the eutectic temperature of 363.degree. C.
According to the electrolytic plating process and the electroless plating process only a polycrystalline film is obtained. Further, no monocrystalline film is obtained, and it is necessary to form a device on the electroconductive surface of gold, copper or nickel.
The processes disclosed in the above-mentioned Japanese Patent Applications Kokai (Laid-open) Nos. 56-38406 and 55-54509 are directed only to formation of gold in a powdery state and nowhere teach or suggest formation of gold in a film state on a desired substrate.
Thus, the technique of forming a gold crystal film including monocrystals, which is applicable to electronic devices, optical devices, etc., on a substrate at a low temperature has room for further improvement.